I'm not sure what is happening here:
https://i.imgur.com/WiNx2IF.png
another example:
https://i.imgur.com/b7jOA2G.png

Made no changes to CC4 Kevin except added beard, 
Double clicked on "Basic T-shirts" and the neck looks messed up.
This seems to happen with other shirts as well.

Doing "Transfer Skin Weights" makes it even worse.
Not sure what I'm doing wrong here.

That is how cloth conform works and that is normal.
Most cloth are developed on neutral male of female characters.
There are a couple of settings like Smooth, Loose, Size.. in Conform, but they do very little in most cases.

Looking at how Kevin's chest is stretched compare to neutral male explains the "phenomena".

If you are looking for perfection, a manual fit would be necessary.
Depending on cloth complexity and given character shape difference (in relation to neutral counterpart),
this could be done by disabling conform for cloth, applying specific character and fitting with Mesh Edit in CC.
Example is here: https://forum.reallusion.com/FindPost540062.aspx

For more complex cases conversion to Accessory and fitting in Blender or such would require.
Such as this one: